Fluid extraction system and related method of controlling operating speeds of electric machines thereof

ABSTRACT

A fluid extraction system is presented. The fluid extraction system includes a direct current (DC) bus and a plurality of fluid extraction sub-systems configured to be electrically coupled to the DC-bus. At least one fluid extraction sub-system includes an electric machine configured to aid in the extraction of a fluid from a well. The electric machine includes a plurality of phase windings and a rotor. The at least one fluid extraction sub-system further includes a control sub-system to control a rotational speed of the rotor by selectively controlling a supply of a phase current to the plurality of phase windings such that the rotational speed of the rotor of the electric machine is different from rotational speed of a rotor of another electric machine in at least one of other fluid extraction sub-systems. Related method for controlling rotational speeds of electric machines is also presented.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 14/939,335 entitled “FLUID EXTRACTION SYSTEM ANDRELATED METHOD OF CONTROLLING OPERATING SPEEDS OF ELECTRIC MACHINESTHEREOF” and filed on Nov. 12, 2015, which is incorporated by referenceherein in its entirety. U.S. Non-Provisional patent application Ser. No.14/939,335 claims the priority and benefit of U.S. ProvisionalApplication No. 62/218,613 entitled “FLUID EXTRACTION SYSTEM AND RELATEDCONTROL SUB-SYSTEM” filed on Sep. 15, 2015, which is incorporated hereinby reference in its entirety.

BACKGROUND

Embodiments of the present disclosure relate to a fluid extractionsystem, and more particularly to a fluid extraction system having acontrol sub-system configured to control a plurality of electricmachines.

In oil and/or gas mining operations, an electric machine, such as anelectric submersible pump (ESP), is prevalently used to extractproduction fluids such as oil and/or gas from a well. The ESP may bedisposed in the well to remove the oil and/or gas. Conventionally, tocontrol an operating parameter such as an operating speed of the ESP, avariable speed drive (VSD) fed by a fixed frequency alternating current(AC) supply is employed. The VSD synthesizes three-phase AC voltages andcurrents of such frequency as is necessary to excite the ESP such thatthe ESP operates in the desired manner.

More particularly, the VSD and a source of the AC supply and aretypically disposed on a surface (e.g., outside the well) and thethree-phase AC power is delivered into the well to the ESP through along cable that extends from the surface to a location inside the wellwhere the ESP is deployed. The supply of the AC power into the well istypically vety costly as more conductors are needed to supply thethree-phase AC power in comparison to the conductors required for thesupply of the DC power.

For increased well productivity, it is desirable to have multipleelectric submersible pumps within the same well, each operable at itsown speed. Accordingly, a well may be formed to have one or morevertical sections and horizontal sections. One or more ESPs may beemployed in each of the vertical sections and horizontal sections.Accordingly, if the abovementioned conventional approach is used, anindividual VSD may be required for each ESP of the one or more ESPs.Moreover, a separate power cable (containing at least three conductorsfor three phase AC power) is required to supply the AC three-phase powerto each ESP. Accordingly, use of such conventional approach is not costeffective.

In another conventional approach, a common VSD is employed to controloperation of the multiple ESPs. However, in such configuration, all theESPs need to operate at the same operating speed. This is unlikely tooptimize the well productivity.

In yet another conventional approach, a single VSD is employed on thesurface that supplies AC power to a first ESP in the well, withsubsequent ESPs being supplied by controllers that are in the well.These controllers need to function as cyclo-converters that receive anAC of one frequency and amplitude into another AC of another frequencyand amplitude for excitation of each additional ESP in the well.However, use of such electronic controllers may not be reliable in theharsh environment within the well. In addition, control of the operatingspeed of an ESP independent of the operation of other ESPs in the well,is another challenge.

BRIEF DESCRIPTION

In accordance with another aspect of the present specification, a fluidextraction system is presented. The fluid extraction system includes adirect current (DC) bus. The fluid extraction system further includes aplurality of fluid extraction sub-systems configured to be electricallycoupled to the DC-bus, where at least one fluid extraction sub-system ofthe plurality of fluid extraction sub-systems includes an electricmachine configured to aid in the extraction of a fluid from a well,where the electric machine includes at least a plurality of phasewindings and a rotor. The at least one fluid extraction sub-systemfurther includes a control sub-system electrically coupled to theelectric machine and configured to control a rotational speed of therotor by selectively controlling a supply of a phase current from theDC-bus to one or more of the plurality of phase windings such that therotational speed of the rotor of the electric machine is different fromrotational speed of a rotor of another electric machine in at least oneof other fluid extraction sub-systems of the plurality of fluidextraction sub-systems.

In accordance with yet another aspect of the present specification, amethod for controlling operating speeds of electric machines in aplurality of fluid-extraction sub-systems coupled to a direct current(DC) bus for receiving a DC voltage is presented. The method includesdetermining a desired operating speed of the electric machine of each ofthe plurality of fluid-extraction sub-systems. The method furtherincludes determining a maximum operating speed of the determined desiredoperating speeds. Furthermore, the method includes adjusting a magnitudeof the DC voltage on the DC-bus based on the maximum operating speedsuch that at least one electric machine of the electric machines isoperable at the maximum operating speed. Moreover, the method alsoincludes generating a speed control signal based on the desiredoperating speed of each electric machine and a desired magnitude of aphase-shift corresponding to the desired operating speed of each of theelectric machine of each of the plurality of fluid-extractionsub-systems. Additionally, the method includes communicating the speedcontrol signal to the plurality of fluid-extraction sub-systems suchthat the electric machine of each of the plurality of fluid-extractionsub-systems are operated at the corresponding desired operating speed.

DRAWINGS

These and other features, aspects, and advantages of the presentspecification will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical illustration of a fluid extraction system, inaccordance with aspects of the present specification;

FIG. 2 is a graphical representation depicting an example relationshipbetween a DC-bus voltage and an operating speed of an electric machine,in accordance with aspects of the present specification;

FIG. 3 is a diagrammatical illustration of a fluid extractionsub-system, in accordance with aspects of the present specification;

FIGS. 4A-4C depict graphical representations of example electric signalsgenerated by the rotor position sensors, in accordance with aspects ofthe present specification;

FIGS. 5A-5C depict graphical representations of example phase shiftedelectric signals, in accordance with aspects of the presentspecification;

FIG. 6 is a graphical representation depicting an example relationshipbetween a phase shift and an operating speed of an electric machine, inaccordance with aspects of the present specification;

FIG. 7 is a flowchart of an example method of controlling operatingspeeds of electric machines in fluid-extraction sub-systems, inaccordance with aspects of the present specification;

FIG. 8 is a flowchart of an example method of generating a speed controlsignal, in accordance with aspects of the present specification; and

FIG. 9 is a flowchart of an example method of controlling an electricmachine, in accordance with aspects of the present specification.

DETAILED DESCRIPTION

The specification may be best understood with reference to the detailedfigures and description set forth herein. Various embodiments aredescribed hereinafter with reference to the figures. However, thoseskilled in the art will readily appreciate that the detailed descriptiongiven herein with respect to these figures is for explanatory purposesas the method and the system may extend beyond the describedembodiments.

In the following specification, the singular forms “a”, “an” and “the”include plural referents unless the context clearly dictates otherwise.As used herein, the term “or” is not meant to be exclusive and refers toat least one of the referenced components being present and includesinstances in which a combination of the referenced components may bepresent, unless the context clearly dictates otherwise.

As used herein, the terms “may” and “may be” indicate a possibility ofan occurrence within a set of circumstances; a possession of a specifiedproperty, characteristic or function; and/or qualify another verb byexpressing one or more of an ability, capability, or possibilityassociated with the qualified verb. Accordingly, usage of “may” and “maybe” indicates that a modified term is apparently appropriate, capable,or suitable for an indicated capacity, function, or usage, while takinginto account that in some circumstances, the modified term may sometimesnot be appropriate, capable, or suitable.

In accordance with some aspects of the present specification, a fluidextraction system is presented. The fluid extraction system includes adirect current (DC) bus and a plurality of fluid extraction sub-systemsdisposed in a well and electrically coupled to the DC-bus. At least onefluid extraction sub-system of the plurality of fluid extractionsub-systems includes an electric machine configured to aid in theextraction of a fluid from a well, where the electric machine includesat least a plurality of phase windings and a rotor. The at least onefluid extraction sub-system further includes a control sub-systemelectrically coupled to the electric machine and configured to control arotational speed of the rotor by selectively controlling a supply of aphase current from the DC-bus to one or more of the plurality of phasewindings such that the rotational speed of the rotor of the electricmachine is different from a rotational speed of a rotor of anotherelectric machine in one or more/at least one other fluid extractionsub-systems of the plurality of fluid extraction sub-systems.

In some embodiments, the control sub-system includes a phase shiftcontrol unit configured to receive an electric signal indicative of anangular position of the rotor from the at least one rotor positionsensor. The phase shift control unit is further configured to generate aphase shifted electric signal by applying a phase shift to the electricsignal, where a magnitude of the phase shift is determined based atleast on a speed control signal indicative of a predetermined rotationalspeed of the rotor. Furthermore, the phase shift control unit is alsoconfigured to generate a phase command signal based on the phase shiftedelectric signal. The control sub-system further includes a switchingunit electrically coupled to the phase shift control unit and configuredto control a supply of a phase current to one or more of the pluralityof phase windings based on the phase command signal such that the rotoris operated at the predetermined rotational speed.

FIG. 1 is a diagrammatical illustration of a fluid extraction system100, in accordance with aspects of the present specification. In oneembodiment, the fluid extraction system 100 may be used to extract fluidfrom a well 102. Examples of the fluid may include, but are not limitedto, water, natural gas, petroleum products such as oil, and the like. Insome embodiments, depending on a resource of the fluid, the well 102 maybe formed of one or more vertical sections such as a vertical section104 and/or one or more horizontal sections such as a first horizontalsection 106 and a second horizontal section 108. In some embodiments,the fluid extraction system 100 may include one or more of a powersource 112, a power converter 114, a DC-bus 116, a plurality of fluidextraction sub-systems 118, and a controller 120.

The power source 112 may be representative of a power generation and/ordistribution system operable to generate and/or distribute analternating current (AC) or a direct current (DC). By way of example,the power source 112 may include an AC generation and/or distributionsystem, a high voltage DC (HVDC) generation and/or distribution system,a medium voltage DC (MVDC) generation and/or distribution system, asolar power plant, a wind based power plant, energy storage mediums suchas batteries, and the like. The power convertor 114 may be an AC-DCconverter or a DC-DC converter.

In some embodiments, the DC-bus 116 may be coupled between the powerconverter 114 and the plurality of fluid extraction sub-systems 118. TheDC-bus 116 may facilitate power transmission from the power source 112to the plurality of fluid extraction sub-systems 118. The DC-bus 116 mayinclude two conductors—one conductor to supply a positive current andanother conductor to supply a negative current, from the power converter114 to the plurality of fluid extraction sub-systems 118.Advantageously, DC transmission to the fluid extraction sub-systems 118in the well 102 results in reduction in overall cost of the fluidextraction system 100 in comparison to AC transmission to the fluidextraction sub-systems 118 in the well 102.

The fluid extraction sub-systems 118 are coupled to the DC-bus 116 andmay aid in extracting the fluid from respective sections of the well102. For example, as depicted in FIG. 1, one fluid extraction sub-system118 is disposed in one or more of the vertical section 104, the firsthorizontal section 106 and the second horizontal section 108. Dependingon a requirement, fewer or greater number of fluid extractionsub-systems 118 may be employed. For simplicity of illustration, onlythe fluid extraction sub-system 118 disposed in the vertical section 104is shown and described in greater details.

The fluid extraction sub-system 118 may include an electric machine 122.In one embodiment, the electric machine 122 may be an electric motor,for example, a permanent magnet (PM) motor. In some embodiments, theelectric machine 122 (for example, the electric motor) may be integratedinto an electric submersible pump (ESP) (not shown in FIG. 1), where theelectric motor may drive an impeller(s) of the ESP. In some embodiments,the electric machine 122 may include a rotor and a stator having aplurality of phase windings (as shown in FIG. 3). Further details of theelectric machine 122 are described in conjunction with FIG. 3.

The fluid extraction sub-system 118 may further include a controlsub-system 124. The control sub-system 124 may be electrically coupledto the electric machine 122 and the DC-bus 116. The control sub-system124 may be controlled by the controller 120. In some embodiments, thecontrol sub-system 124 may be configured to selectively control a supplyof a phase current to the electric machine 122 from the DC-bus 116.Accordingly, the control sub-system 124 may be configured to control arotational speed (for example, revolutions per minute (rpm)) of therotor of the electric machine 122 by selectively controlling the supplyof the phase current to one or more of the plurality of phase windings.In some embodiments, the phase current to one or more of the pluralityof phase windings may be controlled such that the rotational speed ofthe rotor of the electric machine 122 is different from a rotationalspeed of a rotor of another electric machine in one or more other fluidextraction sub-systems 118 of the plurality of fluid extractionsub-systems 118. In some embodiments, the control sub-system 124 may beconfigured to control the rotational speed of the rotor of the electricmachine 122 independent of the operating speeds of rotors in other fluidextraction sub-systems 118. Further details of the control sub-system124 are described in conjunction with FIG. 3. Hereinafter, the terms“rotational speed,” “operating speed,” “operating speed of electricmachine” are interchangeably used and refer to rotational speed in rpmof a rotor of an electric machine, such as the electric machine 122.

The controller 120 may be disposed outside the well 102. The controller120 may include a specially programmed general purpose computer, amicroprocessor, a digital signal processor, and/or a microcontroller.The controller 120 may also include input/output ports and a storagemedium, such as, an electronic memory. Various examples of themicroprocessor include, but are not limited to, a reduced instructionset computing (RISC) architecture type microprocessor or a complexinstruction set computing (CISC) architecture type microprocessor.Further, the microprocessor may be of a single-core type or multi-coretype.

In some embodiments, the controller 120 may be configured to controloperation of the power converter 114 and/or the fluid extractionsub-systems 118 in order to achieve desired flow rate and/or amount ofthe fluid from the well 102. To aid in such control of the powerconverter 114 and/or the fluid extraction sub-systems 118, in oneembodiment, the controller 120 may be coupled to the power converter 114and/or the fluid extraction sub-systems 118 via the DC-bus 116. Inanother embodiment, the controller 120 may be coupled to the powerconverter 114 and/or the fluid extraction sub-systems 118 using separatecables. In yet another embodiment, the controller 120 may be wirelesslycoupled to the power converter 114 and/or the fluid extractionsub-systems 118.

In order to achieve desired flow rate and/or the amount of the fluidfrom the well 102, it may be desired to operate the electric machines ofdifferent fluid extraction sub-systems 118 at optimum operating speeds.Accordingly, it may be desirable to operate the electric machines ofdifferent fluid extraction sub-systems 118 at different operatingspeeds. In some embodiments, the operating speed of the electricmachines used in the fluid extraction sub-systems 118 depends on a DCvoltage on the DC-bus 116 (see FIG. 2).

Turning now to FIG. 2, a graphical representation 200 of an examplerelationship between a DC-bus voltage and an operating speed of anelectric machine is presented, in accordance with aspects of the presentspecification. The X-axis 202 of the graphical representation 200represents time in seconds, for example. The first Y-axis 204 representsDC voltage magnitudes in volts ranging from 0 to 800, for example.Moreover, the second Y-axis 206 represents the operating speed of anelectric machine, such as the electric machine 122, ranging from 0 to1400 rpm, for example. In particular, a graph 208 represents the DCvoltage magnitudes on the DC-bus 116 in steps of 100 volts and a graph210 represents maximum values of the operating speed of the electricmachine 122 corresponding to the magnitudes of DC voltage of the graph208.

Referring again to FIG. 1, in some embodiments, the controller 120 maybe configured with data representative of the graphical representation200. For example, the data representative of the graphicalrepresentation 200 may be stored in the memory associated with thecontroller 120, for example, in the form of a look-up table or amathematical relationship. Accordingly, depending on desired maximumoperating speeds of the electric machines in the fluid extractionsub-systems 118, the controller 120 may be configured to controloperation of the power converter 114 to adjust the magnitude of the DCvoltage on the DC-bus 116. The controller 120 may determine themagnitude of DC voltage based on the data representative of thegraphical representation 200.

In some embodiments, in the absence of the control performed by thecontrol sub-system 124 (as previously noted), the maximum speed of theelectric machines in the fluid extraction sub-systems 118 is defined bythe magnitude of the DC voltage on the DC-bus 116 (see FIG. 2). However,as previously noted, it may be desirable to operate the electricmachines of the fluid extraction sub-systems 118 at different operatingspeeds. Therefore, in certain embodiments, in order to further fine tunethe operating speeds of the electric machines of the fluid extractionsub-systems 118, the controller 120 may be configured to communicate aspeed control signal (described later in FIG. 3) to one or more of thefluid extraction sub-systems 118. The speed control signal may beindicative of desired operating speeds of one or more of the electricmachines in the fluid extraction sub-systems 118. Such speed controlsignal may be received by the control sub-systems of the fluidextraction sub-systems 118. Accordingly, the control sub-systems may beconfigured to further control (for example, reduce) the operating speedof the respective electric machines based on the speed control signal.Consequently, the electric machines of the fluid extraction sub-systems118 may be operated at different operating speeds.

FIG. 3 is a diagrammatical illustration of a fluid extraction sub-system300, in accordance with aspects of the present specification. The fluidextraction sub-system 300 may be representative of one embodiment of thefluid extraction sub-system 118 of FIG. 1.

As previously noted, the fluid extraction sub-system 300 may include anelectric machine such as an electric machine 302 that drives a pump. Theelectric machine 302 may include a stationary stator (not shown in FIG.3). The stator may include a plurality phase windings such as phasewindings 308. The phase windings 308 may be disposed on stator poles(not shown). In some embodiments, as depicted in FIG. 3, the phasewindings 308 may include three sets of windings shown using an alternatepair of ‘dot’ and ‘cross’ symbols. By way of example, the ‘cross’ symbolmay represent winding conductors for supplying a current directed intothe plane of paper and the ‘dot’ may represent winding conductorssupplying the current directed out of the plane the paper.

In some embodiments, the electric machine 302 may include three inputconductors 301, 303, and 305, hereinafter referred to as phase inputlines 301, 303, and 305. The phase input lines 301, 303, and 305 arecoupled to the three sets of the phase windings 308, respectively. Phasecurrent to each of the three set of phase windings 308 may be suppliedvia the phase input lines 301, 303, and 305.

Furthermore, the electric machine 302 may include a rotor 306 movable(for example, rotatable) with respect to the stator when the phasecurrent is supplied to one or more of the phase windings 308. For easeof illustration, the rotor 306 is shown as having two rotor poles 310and 312. Other embodiments of the rotor 306 having greater number ofrotor poles is also contemplated. In one embodiment, the rotor poles 310and 312 may be formed of permanent magnets. By way of example, the rotorpoles 310 and 312 may be disposed such that N-side of the rotor pole 310and S-side of the rotor pole 312 are directed radially outwardly, asdepicted in FIG. 3.

Moreover, in certain embodiments, the electric machine 302 may also beequipped with one or more rotor position sensors such as rotor positionsensors 314, 316, and 318. The rotor position sensors 314-318 may bedisposed proximate to the rotor poles 310 and 312 as depicted in FIG. 3.In the presently contemplated embodiment, the electric machine 302 isshown as having three rotor position sensors 314-318 for ease ofillustration. Use of greater or fewer number of rotor position sensorsis also envisioned, as is determining the rotor position through thevoltages and currents applied to the phase input lines 301, 303, and305. In a non-limiting example, the rotor position sensors 314-318 maybe switches that may transition from ON-state to OFF-state, andvice-versa, based on the sides of rotor poles 310 and 312 that they arefacing. In one embodiment, the rotor position sensors 314-318 transitionto the ON-state when they face rotor poles having their N-sides directedradially outwardly. In other situations, the rotor position sensors314-318 may transition to the OFF-state. For example, in the presentlycontemplated embodiment, the rotor position sensors 314-318 transitionto the ON-state when they face the rotor pole 310 and transition to theOFF-state when they face the rotor pole 312.

Each rotor position sensor of the rotor position sensors 314-318 maygenerate an electric signal (see FIGS. 4A-4C) that is indicative of anangular position of the rotor 306. As previously noted, the rotorposition sensors 314-318 may transition to the ON-state or OFF-statedepending on sides of the rotor poles 310 and 312 that they are facing,which is indicative of the angular position of the rotor 306. In someembodiments, each of the rotor position sensors 314-318, in theON-state, generates an electric signal having a first amplitude (in anon-limiting example, +5 Volts) and, in the OFF-state, generates theelectric signal having a second amplitude (in a non-limiting example, +0Volt). The first amplitude and the second amplitude are hereinafterreferred to as “high level” (H) and “low level” (L), respectively.

FIGS. 4A-4C depict graphical representations of example electric signalsgenerated by the rotor position sensors 314-318, in accordance withaspects of the present specification. In particular, FIG. 4A depicts agraphical representation 402 of an example electric signal 404 generatedby the rotor position sensor 314. The X-axis 406 of the graphicalrepresentation 402 represents a rotor position angle in radians and theY-axis 408 of the graphical representation 402 represents a level of theelectric signal 404. In a non-limiting example, the levels of theelectric signal 404 are represented as “H” and “L.” Similarly, FIG. 4Bdepicts a graphical representation 410 of an example electric signal 412generated by the rotor position sensor 316. The X-axis 414 of thegraphical representation 410 represents the rotor position angle inradians and the Y-axis 416 of the graphical representation 410represents a level of the electric signal 412. In a non-limitingexample, the levels of the electric signal 412 are represented as “H”and “L.” Moreover, FIG. 4C depicts a graphical representation 418 of anexample electric signal 420 generated by the rotor position sensor 318.The X-axis 422 of the graphical representation 418 represents the rotorposition angle in radians and the Y-axis 424 of the graphicalrepresentation 418 represents a level of the electric signal 420. In anon-limiting example, the levels of the electric signal 420 arerepresented as “H” and “L.”

Referring again to FIG. 3, in some embodiments, the fluid extractionsub-system 300 may also include a control sub-system 320 electricallycoupled to the electric machine 302 and configured to control operationof the electric machine 302. Further, the control sub-system 320 mayalso be electrically coupled to the rotor position sensors 314-318. Thecontrol sub-system 320 may receive the electric signals 404, 412, and420 from the rotor position sensors 314, 316, and 318, respectively.

Furthermore, the control sub-system 320 may also be electrically coupledto a DC-bus 322 having a DC conductor 324 carrying a positive DC and aDC conductor 325 carrying a negative DC. Additionally, the controlsub-system 320 may also be configured to receive a speed control signalfrom a controller such as the controller 120 disposed outside the well102. The control sub-system 320 may be configured to selectively controlthe supply of the positive DC and/or the negative DC as phase currentsto the phase windings 308 of the electric machine 302 via the phaseinput lines 301, 303, and 305 based on the speed control signal. Thecontrol sub-system 320 may include a phase-shift control unit 326 and aswitching unit 328. The phase-shift control unit 326 is communicativelycoupled to the switching unit 328.

The phase-shift control unit 326 may include electronics (hardwareand/or software) capable of performing operations including but notlimited to signal decoding and/or delay insertion. In a non-limitingexample, to aid in the above mentioned operations, the phase-shiftcontrol unit 326 may include logic gates, a microprocessor, memory, orcombinations thereof. The microprocessor may include a reducedinstruction set computing (RISC) architecture type microprocessor or acomplex instruction set computing (CISC) architecture typemicroprocessor. Further, the microprocessor may be of a single-core typeor multi-core type.

The phase-shift control unit 326 may be configured to generate a phasecommand signal. The phase command signal may indicate a polarity of thephase current to be supplied to one or more of the phase windings 308.The phase-shift control unit 326 may generate the phase command signalbased on inputs to the phase-shift control unit 326. In someembodiments, the inputs to the phase-shift control unit 326 may includethe electric signals 404, 412, and 420 and the speed control signalreceived from the controller 120 disposed outside the well 102.

In some embodiments, the speed control signal may be in the form of acode. The code may include a plurality of pairs of an identifiersub-code and a speed level sub-code, where each pair of the identifiersub-code and the speed level sub-code may correspond to one of the fluidextraction sub-systems 118 of the fluid extraction system 118. Theidentifier sub-code may be indicative of a fluid extraction sub-systemof the fluid extraction sub-systems 118. The speed level sub-codeassociated with the identifier may be indicative of a desired operatingspeed at which an electric machine of the corresponding fluid extractionsub-system needs to be operated. Details of generation of the speedcontrol signal are described later.

Accordingly, for a given fluid extraction sub-system, such as, the fluidextraction sub-system 300, the phase-shift control unit 326 may beconfigured to decode the speed control signal to determine a speed levelsub-code intended for the fluid extraction sub-system 300. In anon-limiting example, the phase-shift control unit 326 may determine thespeed level sub-code intended for the fluid extraction sub-system 300 bycomparing a pre-defined identifier of the fluid extraction sub-system300 with the identifiers contained in the speed control signal.Accordingly, a speed level sub-code associated with the identifier thatmatches with the pre-defined identifier of the fluid extractionsub-system 300 is selected.

Moreover, the phase-shift control unit 326 may also receive the electricsignals 404, 412, and 420 generated by the rotor position sensors314-318. In order to generate the phase command signal, the phase-shiftcontrol unit 326 may be configured to apply a phase shift to thereceived electric signals to generate phase shifted electric signals(see FIGS. 5A-5C) based on the speed control signal, and moreparticularly, based on the selected speed level sub-code.

FIGS. 5A, 5B, and 5C respectively depict graphical representations 502,504, and 506 of example phase shifted electric signals, in accordancewith aspects of the present specification. The X-axis 508, 510, and 512of the graphical representations 502, 504, and 506, respectively,represent a rotor position angle in radians. The Y-axis 514, 516, and518 of the graphical representations 502, 504, and 506 represents alevel of the phase-shifted electric signals 520, 522, and 524,respectively, the combinations of which determine when correspondingphase input lines conduct phase current. In a non-limiting example, thelevels of the phase-shifted electric signals 520, 522, and 524 arerepresented as “H” and “L.” The phase-shifted electric signals 520, 522,and 524 are representative of the electric signals 404, 412, and 420,respectively, when a phase-shift (for example, a delay) of a magnitude Øis applied to the electric signals 404, 412, and 420, respectively. Itwill be appreciated that the phase-shift may also be negative,corresponding to phase advance rather than phase delay.

Referring again to FIG. 3, in some embodiments, the phase-shift controlunit 326 may be configured to generate the phase-shifted electricsignals 520, 522, and 524 by applying the phase shift of the magnitude Øto the electric signal 404, 412, and 420, respectively, of FIGS. 4A-4C.In some embodiments, the speed level sub-codes contained in the speedcontrol signal are indicative of the different magnitudes of thephase-shifts. Accordingly, the phase-shift control unit 326 maydetermine the magnitude Ø of the phase-shift based on the selected speedlevel sub-code from the speed control signal.

As previously noted, in a fluid extraction system such as the fluidextraction system 100, it may be desirable to operate the electricmachines of the fluid extraction sub-systems 118 at different operatingspeeds. Accordingly, the control sub-systems of the respective fluidextraction sub-systems 118 may determine the magnitude of phase-shiftsbased on their respective pre-defined identifiers and the speed controlsignal in a similar fashion as mentioned above. Accordingly, differentphase-shifts may be applied by different control sub-systems, therebyoperating the electric machines of the fluid extraction sub-systems 118at different operating speeds.

Further, in some embodiments, the phase-shift control unit 326 may beconfigured to generate the phase command signal based on thephase-shifted electric signals 520-524. As previously noted, the phasecommand signal may indicate the polarity of the phase current to besupplied to one or more of the phase windings 308 of the electricmachine 302. Table-1 depicted below illustrates an example relationshipbetween levels of the phase-shifted electric signals 520-524 andpolarity of phase currents to be supplied to the electric machine 302.

TABLE 1 Example relationship between the phase-shifted electric signals520, 522, and 524 and the polarity of phase current Levels of Phase-Phase- Phase- shifted shifted shifted Polarity of Example electricelectric electric phase current phase Row signal signal signal PositiveNegative command No. 520 522 524 Phase line Phase line signals 1 L H L301 303 (+1, −1, 0) 2 L H H 301 305 (+1, 0, −1) 3 L L H 303 305 (0, +1,−1) 4 H L H 303 301 (−1, +1, 0) 5 H L L 305 301 (−1, 0, +1) 6 H H L 305303 (0, −1, +1)

In a non-limiting example, the last column of the Table-1 representsexample codes indicative of the phase command signals generatedcorresponding to the data in the respective row. By way of example, fora given phase command signal (+1, −1, 0), the symbols +1, −1, and 0, arerespectively indicative of the polarity of the phase currents to besupplied on the phase input lines 301, 303, and 305. The symbol +1 mayindicate a positive polarity, the symbol −1 may indicate a negativepolarity, while the symbol 0 may indicate supply of no current.

Moreover, in some embodiments, the phase-shift control unit 326 maycommunicate the generated phase command signal to the switching unit328. The switching unit 328 may be configured to supply the phasecurrents of desired polarities indicated by the phase command signal tothe phase input lines 301, 303, and 305. In order to generate the phasecurrents, the switching unit 328 may include a switch assembly 330 and agate drive unit 332 operatively coupled to the gate drive unit 332. Moreparticularly, output terminals 334, 336, 338, 340, 342, and 344 of thegate drive unit 332 are coupled to the switch assembly 330. The outputterminals 334, 336, 338, 340, 342, and 344 are hereinafter collectivelyreferred to as the output terminals 334-344.

In some embodiments, the switch assembly 330 may include a plurality ofsemiconductor switches for selectively supplying the phase current toone or more of the phase windings 308 based on a plurality of receivedcontrol signals (described later) received from the gate drive unit 332.In a non-limiting example, the switch assembly 330 may include six (6)semiconductor switches 335, 337, 339, 341, 343, 345, arranged as shownin FIG. 3. The semiconductor switches 335, 337, 339, 341, 343, and 345are hereinafter collectively referred to as semiconductor switches335-345. Non-limiting examples of the semiconductor switches 335-345 mayinclude transistors, gate commutated thyristors, field effecttransistors, insulated gate bipolar transistors, gate turn-offthyristors, static induction transistors, static induction thyristors,or combinations thereof. Moreover, materials used to form thesemiconductor switches 335-345 may include, but are not limited to,silicon (Si), silicon carbide (SiC), gallium nitride (GaN), orcombinations thereof.

In a non-limiting example of N-channel field effect transistors, each ofthe semiconductor switches 335-345 may include a drain terminal, asource terminal, and a control terminal (e.g., a gate terminal). Asdepicted in FIG. 3, the drain terminals of the semiconductor switches335, 339, and 343 may be coupled to the DC conductor 324 carrying thepositive DC and the source terminals of the semiconductor switches 337,341, and 345 may be coupled to the DC conductor 325 carrying thepositive DC. Further, the source terminals of the semiconductor switches335, 339, and 343 may be respectively coupled to the drain terminals ofthe semiconductor switches 337, 341, and 345. Furthermore, the sourceterminals of the semiconductor switches 335, 339, and 343 and the drainterminals of the semiconductor switches 337, 341, and 345 may be coupledto the phase input lines 301, 303, and 305 of the electric machine 302.Additionally, the control terminals of the semiconductor switches335-345 are coupled to the gate drive unit 332 to receive the respectiveones of the plurality of control signals. More particularly, the controlterminals of the semiconductor switches 335, 337, 339, 341, 343, and 345are respectively coupled to the output terminals 334, 336, 338, 340,342, 344 of the gate drive unit 332.

In the embodiment of FIG. 3, N-channel MOSFETs are used as thesemiconductor switches 335-345. Accordingly, in one embodiment,application of a control signal of a high level (H) on their controlterminals relative to respective source terminals may turn-on thesemiconductor switches 335-345. Whereas, an application of a controlsignal of a low level (L) on their control terminals relative to therespective source terminals may turn-off the semiconductor switches335-345. It may be noted that if P-channel MOSFETs are used as thesemiconductor switches 335-345, the levels of the control signals may beinterchanged.

The gate drive unit 332 is used to control switching of thesemiconductor switches 335-345 by selectively supplying control signalsto the control terminals of the semiconductor switches 335-345. The gatedrive unit 332 may include electronics (hardware and/or software)capable of generating a plurality of control signals. In a non-limitingexample, the gate drive unit 332 may include logic gates, transistors, amicrocontroller, a microprocessor, memory, or combinations thereof.

The gate drive unit 332 may receive the phase command signal from thephase-shift control unit 326. Subsequent to the receipt of the phasecommand signal, the gate drive unit 332 may be configured to generatethe plurality of control signals based on the phase command signal. Inone embodiment, Table-2 depicted below, represents a plurality ofcontrol signal levels appearing at the output terminals 334-344, wherethe plurality of control signal levels correspond to various phasecommand signals. Moreover, the gate drive unit 332 may be configured tocommunicate the plurality of control signals to the control terminals ofthe semiconductor switches 335-345 via the output terminals 334-344,respectively.

TABLE 2 Example control signal levels appearing at the output terminals334, 336, 338, 340, 342, 344 corresponding to different phase commandsignals Phase command Levels of control signals at signal (334, 336,338, 340, 342, 344) (+1, −1, 0) (H, L, L, H, L, L) (+1, 0, −1) (H, L, L,L, L, H) (0, +1, −1) (L, L, H, L, L, H) (−1, +1, 0) (L, H, H, L, L, L)(−1, 0, −1) (L, H, L, L, H, L) (0, −1, +1) (L, L, L, H, H, L)

A first column of Table-2 represents example phase commands signals andthe second column represents levels of control signals appearing at theoutput terminals 334, 336, 338, 340, 342, and 344, respectively, of thegate drive unit 332. For example, referring to the example illustratedin the first row of Table 2, a phase command (+1, −1, 0) is indicativeof supplying the positive DC to the phase input line 301, supplying thenegative DC to the phase input line 303, and cutting-off supply of thephase current to the phase input line 305. As illustrated in Table-2,the corresponding levels of the control signals at the output terminals334, 336, 338, 340, 342, and 344 are H, L, L, H, L, and L, respectively.Accordingly, the semiconductor switches 335 and 341 are turned-on whileother semiconductor switches (337, 339, 343, and 345) are turned-offConsequently, the positive DC is applied to the phase input line 301 andthe negative DC is applied to the phase input line 303, while no currentis applied to the phase input line 305. Similarly, examples illustratedin other rows of Table 2 may be interpreted likewise.

In some embodiments, the gate drive unit 332 may optionally include apulse width modulation (PWM) sub-unit 333 (shown using dashed box). ThePWM sub-unit 333 may be configured to apply PWM to one or more of thecontrol signals at the output terminals 334, 336, 338, 340, 342, and 344in order to achieve finer control of the operating speed of the electricmachine 302.

In one embodiment, the pulse width of the control signals on one or moreof the output terminals 334, 336, 338, 340, 342, and 344 may becontrolled by the PWM sub-unit 333 based on amplitudes of the phasecurrents on two or more of the phase input lines 301, 303, and 305. Theamplitudes of the phase currents on two or more of the phase input lines301, 303, and 305 may be detected by one or more current sensors (notshown in FIG. 3) electrically coupled to the phase input lines 301, 303,and 305 or disposed in proximity of the phase input lines 301, 303, and305. The one or more current sensors may be disposed within the electricmachine 302 or outside the electric machine 302. Moreover, the one ormore current sensors are electrically coupled to the PWM sub-unit 333for communicating electrical signal indicative of the amplitudes of thephase current on two or more of the phase input lines 301, 303, and 305.

In another embodiment, amplitudes of the phase currents on two or moreof the phase input lines 301, 303, and 305 may be detected by one ormore current sensors (not shown in FIG. 3) electrically coupled to theDC conductors 324 and 325 or disposed in proximity of the DC conductors324 and 325. The one or more current sensors are electrically coupled tothe PWM sub-unit 333 for communicating electrical signal indicative ofthe amplitudes of the phase current on the DC conductors 324 and 325.

FIG. 6 is a graphical representation 600 depicting an examplerelationship between magnitude of a phase shift and an operating speedof an electric machine, in accordance with aspects of the presentspecification. In a non-limiting example, the graphical representation600 depicts the relationship between the magnitude of the phase shiftand the operating speed of the electric machine (for example, 122 or302) at the maximum DC voltage of 800 volts (see FIG. 2). Accordingly,as illustrated a maximum operating speed of 1400 rpm is achievable whenthe DC voltage of 800 volts is provided on the DC-bus 116 (see FIG. 1).

The X-axis 602 of the graphical representation 600 represents time inseconds, for example. The first Y-axis 604 represents the phase-shift indegrees, ranging from −100 to +50, for example. Moreover, the secondY-axis 606 represents the operating speed of the electric machine 122,ranging from 0 to 1400 rpm, for example. A graph 608 is representativeof values of the phase-shifts. Further, a graph 610 indicates values ofthe operating speed of the electric machine 122 corresponding to thevalues of the phase-shifts of graph 608. It may be observed from thegraphical representation 600 that as the phase-shift increases, theoperating speed of the electric machine 122 decreases. For example, atthe DC voltage level of 800 volts on DC-bus 116, the maximum speed thatmay be achieved is 1400 rpm. However, if it is desired to operate theelectric machine 122 at the speed of about 600 rpm, a phase-shift ofabout −32 degrees needs to be introduced in the electric signalgenerated by the rotor position sensors.

In some embodiments, a data representative of the graphicalrepresentation 600 may be stored in the memory associated with thecontroller 120 in the form of a look-up table or a mathematicalrelationship, for example. In certain embodiments, as previously noted,in order to operate the electric machines of the fluid extractionsub-systems 118 at different operating speeds, the controller 120 maygenerate and communicate the speed control signal to the fluidextraction sub-systems 118. In a non-limiting example, the controller120 may generate the speed control signal based on the relationshipdepicted in the graphical representation 600.

By way of example, during operation of the fluid extraction system 100,for a given DC voltage magnitude (for example, 800 volts), if oneelectric machine (for example, the electric machine 122) of the fluidextraction sub-systems 118 in the vertical section 104 is desired to beoperated at speed of 600 rpm, the controller 120 may determine that aphase-shift of −32 degrees needs to be introduced in the electric signalgenerated by the rotor position sensors. Similarly, depending on thedesired operating speeds of electric machines in other fluid extractionsub-systems 118, the controller 120 may determine the magnitudes ofrequired phase-shifts. Once the magnitudes of required phase-shifts areidentified, the controller 120 may determine the speed level sub-codescorresponding to the determined magnitudes of required phase-shifts. Insome embodiments, a mapping of the magnitudes of phase-shift and speedlevel sub-codes is stored in the memory associated with the controller120. Accordingly, the controller 120 may generate speed level sub-codescorresponding to the magnitudes of required phase-shifts using themapping of the magnitudes of phase-shift and speed level sub-codesstored in the memory. Once the speed level sub-codes are generated, thecontroller 120 may generate the speed control signal having the pairs ofthe identifier sub-code and the speed level sub-code for the fluidextraction sub-systems 118 of the fluid extraction system 100. Thecontroller 120 may further be configured to communicate the generatedspeed control signal to the fluid extraction sub-systems 118.

FIG. 7 is a flowchart 700 of an example method of controlling theoperating speeds of electric machines in the fluid extractionsub-systems 118 of FIG. 1 coupled to the DC-bus 116, in accordance withaspects of the present specification. More particularly, the controller120, by executing the method of flowchart 700, may be configured tocontrol the operating speed of a given electric machine, such as theelectric machine 122, independent of the operating speeds of electricmachines of other fluid extraction sub-systems 118.

In some embodiments, it may be desirable to operate the electricmachines of the fluid extraction sub-systems 118 at different operatingspeeds. Accordingly, at step 702, desired operating speeds ofcorresponding electric machines of one or more of the plurality of fluidextraction sub-systems 118 are determined. The controller 120 maydetermine the desired operating speeds of the electric machines of theone or more of the plurality of fluid extraction sub-systems 118 basedon a type, desired flow and/or amount of the fluid to be extracted.Further, a maximum operating speed of the determined operating speedsmay be determined by the controller 120, as indicated by step 704.

Furthermore, at step 706, a magnitude of the DC voltage on the DC-bus116 may be adjusted by the controller 120. In some embodiments, themagnitude of the DC voltage on the DC-bus 116 may be adjusted based onthe maximum operating speed such that at least one electric machine ofthe electric machines may be operable at the maximum operating speed. Aspreviously noted, the controller 120 may determine the magnitude of theDC voltage to appear on the DC-bus 116 based on the relationship asdepicted in the graphical representation 200 of FIG. 2.

Moreover, at step 708, a speed control signal may be generated by thecontroller 120 based on the desired operating speed of each electricmachine and a desired magnitude of a phase-shift corresponding to thedesired operating speed of each of the electric machine of each of theplurality of fluid extraction sub-systems 118. Details of generating thespeed control signal are described in conjunction with FIG. 8. Aspreviously noted, the speed control signal may be in the form of a code.The code may include a plurality of pairs of an identifier sub-code anda speed level sub-code, where each pair of the identifier sub-code andthe speed level sub-code may correspond to one of the fluid extractionsub-systems 118 of the fluid extraction system 118. The identifiersub-code may be indicative of a fluid extraction sub-system of the fluidextraction sub-systems 118. The speed level sub-code may be indicativeof a desired operating speed at which an electric machine of thecorresponding fluid extraction sub-system needs to be operated.

Additionally, the speed control signal may be communicated to the fluidextraction sub-systems 118 by the controller 120, as indicated by step710. More particularly, the speed control signal may be communicated sothat the electric machines in the fluid extraction sub-systems 118 maybe operated at the corresponding desired operating speeds. In someembodiments, in each fluid extraction sub-system 300, the speed controlsignal may be received by a respective phase-shift control unit, such asthe phase-shift control unit 326. Detailed steps for controllingoperation of an electric machine are described in conjunction with FIG.9.

FIG. 8 is a flowchart 800 of an example method of generating speedcontrol signal, in accordance with aspects of the present specification.As previously noted, at step 702 of flowchart 700, the controller 120may have determined the desired operating speeds of the electricmachines of the fluid extraction sub-systems 118. Also, magnitude of theDC voltage may be adjusted at a determined level at step 706 of theflowchart 700.

At step 802, a magnitude of phase-shift corresponding to the desiredoperating speed of the electric machine of each of the plurality offluid extraction sub-systems 118 is determined by the controller 120. Aspreviously noted, the controller 120 may determine the magnitudes ofphase-shifts corresponding to the desired operating speeds using therelationship depicted in the graphical representation 600 of FIG. 6.

Moreover, a speed level sub-code corresponding to each of the determinedmagnitudes of the phase-shifts may be generated by the controller 120,as indicated by step 804. The controller 120 may generate speed levelsub-codes corresponding to the magnitudes of required phase-shifts usinga mapping of the magnitudes of phase-shift and speed level sub-codes,stored in the memory associated with the controller 120.

In some embodiments, the memory associated with the controller 120 alsoincludes identifier sub-codes of the plurality of fluid extractionsub-systems 118. Accordingly, once the speed level sub-codes aregenerated, the speed control signal in the form of a code may begenerated by the controller 120, as indicated by step 806. By way ofexample, the speed control signal may be generated by concatenatingpairs of the identifier sub-code and the speed level sub-code for eachof the fluid extraction sub-systems 118.

FIG. 9 is a flowchart 900 of an example method of controlling anelectric machine such as the electric machine 302 (see FIG. 3), inaccordance with aspects of the present specification.

At step 902, a speed control signal, such as, the speed control signalgenerated at step 706 (see FIG. 7), may be received by the controlsub-system 320 of the fluid extraction sub-system 300. Moreparticularly, the speed control signal may be received by thephase-shift control unit 326. Further, an electric signal indicative ofan angular position of the rotor 306 may be received by the phase-shiftcontrol unit 326, as indicated by step 904. By way of example, theelectric signals 404, 412, and 420 (see FIG. 4) generated by the rotorposition sensors 314-318 may be received by the phase-shift control unit326.

Furthermore, at step 906, a phase shifted electric signal may begenerated by applying a phase shift to the electric signal. For example,the phase-shift control unit 326 may generate the phase-shifted signals520-524 (see FIG. 5) by applying a phase-shift to the electric signals404, 412, and 420. In one embodiment, a magnitude of the phase shift isdetermined based at least on the speed control signal indicative of thedesired rotational speed (e.g., the operating speed) of the rotor 306,More particularly, the controller 120 may determine the magnitude of thephase shift based on the speed level sub-code (contained in the speedcontrol signal) corresponding to the fluid extraction sub-system 300.

Additionally, a phase command signal may be generated by the phase-shiftcontrol unit 326 based on the phase-shifted electric signals 520-524, asindicated by step 908. In one embodiment, the phase command signal maybe generated by the phase-shift control unit 326 based on the levels ofthe phase-shifted electric signals 520-524 using the relationshipdepicted in Table-1.

Moreover, at step 910, a plurality of control signals may be generatedbased on the phase command signal. The plurality of control signals isgenerated by the gate drive unit 332 using, for example, therelationships of Table-2. Depending on the plurality of the controlsignals, a phase current may be selectively supplied to one or more ofthe plurality of phase windings 308, as indicated by step 912. Further,based on the plurality of control signals, one or more of thesemiconductor switches 334-344 may be selectively turned-on to supplythe phase current with polarities indicated by the phase command signal.

Any of the foregoing steps and/or system elements may be suitablyreplaced, reordered, or removed, and additional steps and/or systemelements may be inserted, depending on the needs of a particularapplication, and that the systems of the foregoing embodiments may beimplemented using a wide variety of suitable processes and systemelements and are not limited to any particular computer hardware,software, middleware, firmware, microcode, and the like.

Furthermore, the foregoing examples, demonstrations, and method stepssuch as those that may be performed by the controller 120 may beimplemented by suitable code on a processor-based system, such as ageneral-purpose or special-purpose computer. Different implementationsof the systems and methods may perform some or all of the stepsdescribed herein in different orders, parallel, or substantiallyconcurrently. Furthermore, the functions may be implemented in a varietyof programming languages, including but not limited to C++ or Java. Suchcode may be stored or adapted for storage on one or more tangible,computer readable media, such as on data repository chips, local orremote hard disks, optical disks (that is, CDs or DVDs), memory or othermedia, which may be accessed by a processor-based system to execute thestored code.

The systems and methods described herein aids in reducing overall costof the fluid extraction system. The cost effectiveness is achieved atleast in part due to the use of the supply of variable DC voltage to thefluid extraction sub-systems in the well. Further, the operating speedsof the electric machines are independently controllable. Such anindependent control of the operating speeds is achieved at least in partby the use of the speed control signal and its decoding by thephase-shift control unit to control the supply of the phase currents tothe electric machine. Moreover, use of the semiconductor switches thatare formed using material that are capable of withstanding hightemperatures, such as silicon carbide, aids in improving reliability.

It will be appreciated that variants of the above disclosed and otherfeatures and functions, or alternatives thereof, may be combined tocreate many other different systems or applications. Variousunanticipated alternatives, modifications, variations, or improvementstherein may be subsequently made by those skilled in the art and arealso intended to be encompassed by the following embodiments.

1. A fluid extraction system, comprising: a direct current (DC) bus; anda plurality of fluid extraction sub-systems configured to beelectrically coupled to the DC-bus, wherein at least one fluidextraction sub-system of the plurality of fluid extraction sub-systemscomprises: an electric machine configured to aid in the extraction of afluid from a well, wherein the electric machine comprises at least aplurality of phase windings and a rotor; and a control sub-systemelectrically coupled to the electric machine and configured to control arotational speed of the rotor by selectively controlling a supply of aphase current from the DC-bus to one or more of the plurality of phasewindings such that the rotational speed of the rotor of the electricmachine is different from rotational speed of a rotor of anotherelectric machine in at least one of other fluid extraction sub-systemsof the plurality of fluid extraction sub-systems.
 2. The fluidextraction system of claim 1, wherein the electric machine is anelectric motor integrated into an electric submersible pump (ESP). 3.The fluid extraction system of claim 2, wherein the electric motor is apermanent magnet motor.
 4. The fluid extraction system of claim 1,wherein the at least one fluid extraction sub-system further comprisesat least one rotor position sensor disposed proximate to the rotor andconfigured to generate an electric signal indicative of an angularposition of the rotor.
 5. The fluid extraction system of claim 4,wherein the control sub-system of the at least one fluid extractionsub-system comprises a phase shift control unit configured to: receivethe electric signal from the at least one rotor position sensor;generate a phase shifted electric signal by applying a phase shift tothe electric signal, wherein a magnitude of the phase shift isdetermined based at least on a speed control signal indicative of therotational speed of the rotor; and generate a phase command signal basedon the phase shifted electric signal, wherein the phase command signalis indicative of a polarity of the phase current to be applied to one ormore of the plurality of phase windings.
 6. The fluid extraction systemof claim 5, wherein the control sub-system of the at least one fluidextraction sub-system further comprises a switching unit electricallycoupled to the phase shift control unit and the DC-bus and configured toselectively control the supply of the phase current to one or more ofthe plurality of phase windings based on the phase command signal. 7.The fluid extraction system of claim 6, wherein the switching unitcomprises a gate drive unit electrically coupled to the phase shiftcontrol unit and configured to generate a plurality of control signalsbased on the phase command signal.
 8. The fluid extraction system ofclaim 7, wherein the switching unit further comprises a switchingassembly having a plurality of semiconductor switches for selectivelysupplying the phase current to one or more of the plurality of phasewindings based on the plurality of control signals.
 9. The fluidextraction system of claim 8, wherein each of the plurality ofsemiconductor switches comprises a control terminal, wherein the controlterminal is coupled to the gate drive unit for receiving a correspondingcontrol signal of the plurality of control signals.
 10. The fluidextraction system of claim 5, further comprising a controller disposedoutside the well and configured to transmit the speed control signal tothe phase shift control unit.
 11. The fluid extraction system of claim10, further comprising a power converter disposed outside the well andoperatively coupled to the controller and configured to provide a DCpower on the DC-bus based on a power control signal from the controller.12. The fluid extraction system of claim 1, wherein a maximum rotationalspeed of the rotor in the electric machine depends on a DC voltage levelof the DC-bus.
 13. The fluid extraction system of claim 1, wherein theplurality of fluid extraction sub-systems comprises a first set of fluidextraction sub-systems and a second set of fluid extraction sub-systems,wherein the first set of fluid extraction sub-systems is disposed in afirst horizontal section of the well and the second set of fluidextraction sub-systems is disposed in a second horizontal section of thewell.
 14. The fluid extraction system of claim 1, wherein the pluralityof fluid extraction sub-systems comprises a third set of fluidextraction sub-systems disposed in a vertical section of the well.